NEW YORK (Reuters Health) - A problem shoulder might shorten the career of college athletes who are talented enough to get drafted by a professional football team, according to a new study.

The study focused on college players drafted into National Football League (NFL) teams. By the time they were drafted, all of the athletes had already had shoulder dislocations serious enough to require surgery to stabilize the joint.

Those who went on to become linemen and linebackers played more than 30 percent fewer games and years compared to similar draftees who hadn't had the shoulder operations.

Whether the bum shoulders were truly to blame for lost field time is not completely clear, however, because the researchers could only review players' careers in retrospect, with incomplete data on other aspects of their lives.

Shoulder instability resulting from one or more dislocations is common in contact sports. The socket of the shoulder joint is very shallow; the top of the upper arm bone (the humerus) rests on the shoulder blade like a golf ball on a tee. Cartilage and ligaments help keep everything in place - but a hard hit or fall can cause the golf ball to come off the tee, dislocating the shoulder.

"Not everyone who suffers a dislocation will undergo stabilization surgery, but the vast majority do, particularly if they dislocate it more than once," lead researcher Dr. Robert H. Brophy of the Washington University School of Medicine in St. Louis, Missouri, told Reuters Health.

Before the draft, the NFL invites the top college athletes to an intense multi-day event known as the Scouting Combine. There, the teams evaluate players' football skills and physical condition. Brophy and colleagues say almost 10 percent of athletes arrive at the combine with a history of shoulder instability and nearly five percent have had the surgery.

To see how such a medical record might affect a player's professional career, Brophy and his colleagues identified 42 NFL players who had shoulder stabilization surgery before joining the league between 1987 and 2000. Each athlete was then matched to a player without a history of the surgery but who shared other factors that might influence a football career: position, year of entry into the NFL and round chosen in the draft, a rough measure of talent.

Shoulder stabilization surgery seemed to impact the careers of "guys who play on the line and linebackers," Brophy said.

That would make sense, he added, as players in these positions are involved in a lot of blocking and pushing, as well as intense strength training, all of which put the shoulder at risk.

They found that the average career length of linemen and linebackers who had the surgery was significantly shorter than comparable players who had not: 4.7 versus 6.7 years. A similar pattern was seen for the average number of career games played: 51 versus 81.

For men playing other positions, the researchers couldn't prove an effect of the surgery. It's possible, however, that the study was too small, and that if they'd studied more athletes an effect might have become obvious.

While the researchers can't be certain if the shoulder injury and surgery are directly curtailing careers, they do offer a few potential explanations for a link. For example, there might be a higher risk of posttraumatic arthritis over time in the damaged shoulder. Recurrent episodes of instability may also directly increase the amount of time players spend on the sidelines and could even lead to the loss of a starting position or job, the researchers report in The American Journal of Sports Medicine.

They also point out that surgical methods have improved since most of the players in the study had their operations. More than 90 percent had "open" surgery involving large incisions, but more recently there's been a shift toward performing stabilization surgery arthroscopically, which is less invasive. Whether the modern surgery is better or worse for a career, however, remains unclear.

Unfortunately, due to the traumatic nature of the injury, there is no good prevention strategy. "Most of the guys already do a good job of maintaining their larger muscle groups, as well as the smaller muscles of their shoulder," said Brophy.

"The good news is that this isn't a career-ending injury," he added. "People can recover and play. But, on average, it certainly does seem to affect how long these guys can compete at the highest level."